Drop oauth2client as a submodule.

Previously, we'd kept `oauth2client` as a submodule, so that we could easily
track changes at HEAD. Going forward, we won't need to do this, so we switch
to having an explicit dependency in `setup.py`.
Add python 2.6 to tox testing.
Update author info.
Add apiclient as an alias for googleapiclient.

We want to rename `apiclient` to `googleapiclient`; breaking everyone seems
like a **bad** way to do that. Instead, this keeps the old name as an alias.
Add support for local discovery doc to sample_tools.py.

This patch (originally based on one by @jaybeeo) updates sample_tools to also
accept a local discovery file, and avoids calling out to the discovery service
in that case.
Version 1.3 release.

Updates CHANGELOG, version, docs.
Install argparse on python < 2.7.
Add __version__ to apiclient.
Clean up imports in apiclient/__init__.py.
Add detection for old apiclient versions in setup.py.
Update CHANGELOG for release.
Add another check for a failed googleapiclient upgrade.

This adds one more check for a failed 1.2 -> 1.3 upgrade, specifically in the
`apiclient` import (which was the only import available in 1.2). Even combined
with the check in setup.py, this won't catch everything, but it now covers all
the most common cases.
Official v1.3 release.

(Updates docs.)